The long term goal of this project is to explore the physiology and functional roles of the intrinsically photosensitive retinal ganglion cells (ipRGCs). The present proposal is to investigate the interactions of ipRGCs with key processes in the developing retina. The ipRGCs are the first functional photoreceptors of the mammalian retina, generating electrical responses to light more than a week before rod and cone photoreceptors are mature enough to affect retinal output. At this age, ganglion cell axons are already establishing and refining their central projections to the visual centers of the brain. This process is thought to be dependent on retinal activity, especially the waves of electrical activity that sweep across the inner retina. During a critical developmental stage (first postnatal week in mice), retinal waves are driven by a network of cholinergic (starburst) amacrine cells which excite each other as well as ganglion cells through nicotinic receptors. These Stage II retinal waves have been considered immune from photic influence due to the immaturity of the classical photoreceptors. However, our preliminary evidence shows that light does, in fact, modulate the behavior of Stage II retinal waves and this influence requires melanopsin, the photopigment of ipRGCs. In return, the waves excite ipRGCs. These bidirectional interactions between retinal waves and ipRGCs are unexpected, and have significant implications for visual system development. The central focus of this renewal application is to explore the nature, mechanisms and functional implications of the bidirectional interactions between ipRGCs and Stage II retinal waves. The specific aims of the proposal are: 1) to determine the synaptic mechanisms by which waves excite melanopsin ganglion cells and how the waves shape the central projections of ipRGCs; and 2) to assess the impact of ipRGCs on retinal waves, the mechanisms responsible for these effects, and their impact on development of retinal projections to central visual targets. Proposed studies will be conducted in wildtype and genetically modified mice and will involve in vitro recordings and pharmacological manipulation of retinal neurons; gene expression profiling; and tracing of retinofugal projections. These studies will help to document an important and novel functional role for ganglion cell photoreceptors, and will clarify mechanisms responsible for their surprising influence on other retinal neurons. They will refine our understanding of the role of lightdriven activity in visual system development and may prompt a reconsideration of the possible impact of lighting environments on visual system development in premature human infants.